The essential parts of a sand filter are the water reservoir, filter bed and drainage system, all contained within the filter box. A filter cover may also be required.
The filter bed and drainage system consist of layers of gravel and sand. Water is first passed through gravel bed to remove larger pieces of contaminants, and then it passes through the sand bed, which removes smaller pieces. After being filtered, the water leaves the system through a spout into a reservoir. A chemical coagulant is sometimes added to the sand portion of the filter to increase the adhesion of particles to the sand.